APPARATUS AND METHOD FOR OSCILLATING FLUID JETS

RELATED APPLICATIONS The present application claims the benefit of US Provisional Patent Applications

60/924,157 and 60/924,158 both filed on May 2, 2007 and is related to co-pending application entitled "METHODS AND APPARATUS FOR REDUCTION OF AERODYNAMIC DRAG" by G Arwatz, I Fono and A Seifert identified as attorney docket number 43724 and filed as a PCT Patent Application the same day as the instant application The disclosures of all of the above mentioned applications are fully incorporated herein by reference

FIELD OF THE INVENTION

The present invention relates to oscillating fluid jets, methods of production thereof and apparatus for producing same BACKGROUND OF THE INVENTION

Flow control technology relates generally to the capability to alter flow properties relative to their natural tendencies by introduction of a constant, or periodic, excitation Use of a periodic excitation for control of boundary layer separation has been demonstrated to be both possible and efficient in incompressible flows (Seifert et al (1996) "Delay of Airfoil Stall by Periodic Excitation", J of Aircraft VoI 33, No 4, pp 691 699 and Seifert et al (1999) "Oscillatory Control of Separation at High Reynolds Numbers", AIAA J 37(9) 1062-1071) especially at low speeds and in a wide range of Reynolds numbers (Re, 10 ⁴ to 10 ⁷ ) Control of boundary layer separation in compressible flows has also been demonstrated, although the level of oscillation required is higher than that required in in-compressible flows (Seifert et al (2001 ) "Oscillatory Control of Shock-induced Separation", (AIAA paper 99 0925), J Aircraft, 38(3) 464 472 and Seifert et al (2003) "Effects of Compressibility and Excitation Slot Location on Active Separation Control at High Reynolds Numbers", J Aircraft 40 (1) pp 110- 119) Despite this, as long as the flow is free of shock waves, there is no theoretical or physical difference resulting from the mere increase of Mach number One of the primary uses of flow control in boundary layer control is to delay, prevent or manage unwanted boundary layer separation

Significant scientific and technological effort has been invested in control of boundary layer separation Alternate methods of flow actuation have been examined including mechanical mixing (e g vortex generators, Allan et al (2002) Numerical Simulations of Vortex Generator Vanes and Jets on a Flat Plate, AIAA Paper 2002 3160), pneumatic vortex generator-jets (e g , steady and oscillatory, Johnston, et al (2002) International J of Heat and Fluid Flow, 23(6) 750 757 , and Khan and Johnston, (2000) International J of Heat and Fluid Flow, (21(5) 505 511 ) and cyclic excitation Under certain conditions (e g at low Re numbers) that cyclic excitation is

more efficient than steady excitation for boundary layer control by about two orders of magnitude (Seifert et al (1996) J of Aircraft 33(4) 691-699)

Prandtl defined the boundary layer and the scientific and engineering advantages to be realized its control Prandtl also defined the basic theoretical problems related to control of boundary layer separation and went on to explain one possible solution to these problems, control of the boundary layer separation by suction, applied upstream of the separation point with suppression of the negative phenomena resulting from the flow detachment from the surface These phenomena lead to reduction in efficiency of the flow related mechanism Prandtl demonstrated the efficacy of boundary layer suction by placement of suction ports upstream to the boundary layer separation point in a wide angle diffuser, whose boundary layers separated without control In the presence of suction, the flow remained attached to the two walls of the diffuser (Prandtl and Teitjens (1934) Applied Hydro and Aerodynamics, Dover, NY, page 294)

Even in a case where suction of the boundary layer prevents separation locally, downstream spreading of flow streamlines can cause boundary layer separation downstream of the point where suction is applied

US Patent 7,055,541 to Seifert et al describes methods and mechanisms for Producing Suction and Periodic Excitation Flow including embodiments in which an exit flow direction oscillates of a boundary layer control fluid stream oscillates The disclosure of this patent is fully incorporated herein by reference

It is known to employ a converging-diverging inlet nozzle in conjunction with a suction flow In an apparatus with a given set of dimensions, a converging-diverging inlet nozzle can function in a variety of ways depending on operational conditions (see Fig 3, taken from Streeter and Wylie (1981 ) Fluid Mechanics, 7th Edition, McGraw Hill Ryerson, page 283) Fig 3 presents converging-diverging nozzle pressure and Mach number characteristics If the flow at the nozzle exit is desired to be supersonic, it is required to have a nozzle pressure-ratio that will ensure ideally expanded jet (below point j, in Fig 3) A higher pressure ratio will cause a shock wave right downstream of the nozzle exit and the flow will return to subsonic conditions When wholly subsonic operation is desired and efficiency is a prime consideration, a short converging inlet nozzle suffices

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates to oscillation control ports connected by a feedback tube In an exemplary embodiment of the invention, the ports are provided transverse to a wall of a conduit through which a fluid flow is directed Optionally, the fluid flow oscillates automatically between two or more exit directions as a result of flowing past the oscillation control ports

In an exemplary embodiment of the invention, a primary flow of fluid though the conduit past one of the oscillation control ports causes a negative control pressure in one of the oscillation control ports which propagates through the feedback tube and causes a positive

control pressure at the other control port The positive control pressure diverts the fluid flow towards the other control port Iterative repetition of this process causes the flow to oscillate Optionally, the conduit is characterized by two or more defined exit directions and oscillation is between the defined exit directions Optionally, the primary flow is amplified by allowing additional fluid to join the primary flow through one or more suction ports in fluid communication with the conduit upstream of the oscillation control ports

According to various exemplary embodiments of the invention, oscillation frequency is determined by one or more of a flow rate through the conduit, a dimension of the feedback tube and a dimension of the conduit

In an exemplary embodiment of the invention, a fluid flow through a conduit characterized by two exit directions passes oscillation control ports transverse to a wall of the conduit and connected to one another by a feedback tube Optionally, the feedback tube is adapted to control oscillation Optionally, the fluid flow is directed towards a first exit direction and creates a negative pressure at a first oscillation control port Optionally, Bernoulli's principle contributes to creation of the negative pressure

The negative pressure produces a secondary flow towards a first oscillation control port in the feedback tube In an exemplary embodiment of the invention, the secondary flow in the feedback tube causes a positive pressure in a second oscillation control port Optionally, the positive pressure in the second oscillation control port the fluid flow to shift to a second exit direction

Once the fluid flow is in the second exit direction, it creates a negative pressure at the second oscillation control port and a direction of secondary flow in the feedback tube is reversed In an exemplary embodiment of the invention, this process is iteratively repeated and results in an oscillation of the primary flow between the exit directions Optionally, a frequency of the oscillation can be controlled by altering one or more dimensions of the feedback tube and/or the oscillation control ports and/or the conduit during design Alternatively, or additionally, a rate of the fluid flow contributes to oscillation frequency

In an exemplary embodiment of the invention, feedback control is dynamic Optionally, two or more feedback tubes are provided, with switches to put one or more into/out of play

Optionally, oscillation can be interrupted or prevented by blocking a flow through the feedback tube

In an exemplary embodiment of the invention, the apparatus is incorporated into a system with a controller Optionally, the controller includes one or more of a flow regulator, a switching mechanism and a dimension changer

In an exemplary embodiment of the invention, there is provided a method of causing a fluid flow to oscillate between two exit directions, the method comprising (a) causing a primary flow of fluid though a conduit, the conduit characterized by two exit directions, and

(b) providing first and second oscillation control ports, said first and second oscillation control ports transverse to said conduit and connected to one another by a feedback tube

Optionally, the method comprises,

(c) amplifying the primary flow by allowing additional fluid to join said flow through at least one suction port in fluid communication with said conduit upstream of the oscillation control ports to create an amplified flow

Optionally, the method comprises, varying a frequency of oscillation by altering a length of the feedback tube

Optionally, the method comprises, interrupting oscillation by preventing a secondary flow through the feedback tube

Optionally, the method comprises, varying a frequency of oscillation by altering a rate of the primary flow

Optionally, the method comprises, regulating a frequency of oscillation by controlling a diameter of the feedback tube Optionally, the method comprises, regulating a frequency of oscillation by controlling a volume of the feedback tube

Optionally, the method comprises, regulating a frequency of oscillation by controlling a conduit dimension

Optionally, the method comprises, regulating a frequency of oscillation by controlling a control port dimension

Optionally, the causing comprises directing the primary flow through a converging diverging nozzle

In an exemplary embodiment of the invention, there is provided an automatic mechanism to produce a fluid jet with an oscillating exit direction, the mechanism comprising (a) a conduit adapted to convey a flow of fluid, the conduit characterized by two exit directions, and

(b) a feedback control tube terminating in first and second oscillation control ports, said first and second oscillation control ports transverse to a wall of said conduit and connected to one another by the feedback tube Optionally, the apparatus comprises, a jet port adapted to direct the flow into the conduit at a controlled input pressure

Optionally, the jet port comprises a converging-diverging nozzle

Optionally, the apparatus comprises, at least one suction port in fluid communication with said conduit and an environment external to the mechanism, said at least one suction slot capable of allowing additional fluid to join said flow to create an amplified flow

Optionally, the apparatus is adapted for automatic oscillation based solely on energy from the flow

Optionally, the apparatus comprises an oscillation regulation mechanism

Optionally, the oscillation regulation mechanism is adapted to vary a frequency of oscillation by altering a volume of the feedback tube

Optionally, the oscillation regulation mechanism is adapted to vary a flow of fluid through the conduit Optionally, the apparatus comprises, an oscillation interruption mechanism adapted to prevent a control pressure through the feedback tube

Optionally, the interruption mechanism seals the tube

BRIEF DESCRIPTION OF THE DRAWINGS Exemplary non-limiting embodiments of the invention described in the following description, read with reference to the figures attached hereto In the figures, identical and similar structures, elements or parts thereof that appear in more than one figure are generally labeled with the same or similar references in the figures in which they appear Dimensions of components and features shown in the figures are chosen primarily for convenience and clarity of presentation and are not necessarily to scale The attached figures are

Fig 1 is a schematic representation of an apparatus according to an exemplary embodiment of the invention illustrating connection of control ports by a feedback tube according to an exemplary embodiment of the invention,

Fig 2 is a simplified flow diagram of a method according to an exemplary embodiment of the invention,

Fig 3 (prior art) illustrates various pressure and Mach number configurations of flow through a converging-diverging nozzle,

Fig 4a is a graph of oscillation frequency (Hz) as a function of inlet flow rate (Liters/sec) according to various exemplary embodiments of the invention, Fig 4b is a graph of oscillation frequency (Hz) as a function of feedback tube length

(M) according to various exemplary embodiments of the invention,

Fig 5 is a graph of oscillation frequency (Hz) as a function of inlet flow rate (Liters/sec) according to various exemplary embodiments of the invention,

Fig 6 is a schematic representation of an apparatus according to an exemplary embodiment of the invention featuring an exemplary converging-diverging nozzle as an inlet port,

Fig 7 is a schematic representation of an exemplary system including an apparatus according to an exemplary embodiment of the invention and an exemplary controller, and

Fig 8 is a schematic representation of a tube switching mechanism for use in the context of an exemplary embodiment of the invention

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Overview

The invention relates generally to oscillating fluid jets, methods of production thereof and apparatus for producing same In an exemplary embodiment of the invention, oscillation is

automatic Optionally, an oscillation frequency can be controlled by altering an inlet flow rate, amplified flow rate and/or apparatus dimensions In an exemplary embodiment of the invention, flow rate is amplified by means of suction ports which draw additional fluid into the system In an exemplary embodiment of the invention, the additional fluid mixes with an inlet flow to create an amplified flow Exemplary embodiments of the invention share, as a common feature, a pressure operated oscillation control mechanism In an exemplary embodiment of the invention, the oscillation control mechanism comprises two oscillation control ports connected by a feedback tube

Fig 2 is a simplified flow diagram of an exemplary method 200 of causing a fluid flow in a conduit to oscillate between two exit directions according to an embodiment of the invention Figs 1 and 6 illustrate exemplary apparatus adapted to cause a fluid flow in a conduit to oscillate between two exit directions according to various embodiments of the invention Exemplary Method

Referring now concurrently to Figs 1 and 2 at 210 (Fig 2) an injection port 310 (Fig 1 ) causes a primary fluid flow 332 though a conduit 330 characterized by two exit directions 350 and 360

In order to induce oscillation, oscillation control ports 340 are provided (220) transverse to a wall of conduit 330 In an exemplary embodiment of the invention, oscillation control ports 340 are connected (230) to one another by a feedback tube 440 adapted to control oscillation In an exemplary embodiment of the invention, primary flow 332a is directed towards a first exit direction 350 Optionally, flow through the conduit is bi-stable so that either 332a or 332 b can occur, but a neutral state in which 332a and 332b occur together is unstable Flow 332a creates (240) a negative pressure 344 (ι e out of port 340) at a first (upper in this view) oscillation control port 340 Optionally, Bernoulli's principle contributes to creation of negative pressure 344

Negative pressure 344 produces (250) a secondary flow in a feedback tube 440 (Fig 1 ) towards first oscillation control port 340 In an exemplary embodiment of the invention, the secondary flow (indicated as a series of hollow arrows in Fig 1 ) in the feedback tube causes (260) a positive pressure 448 in a second (bottom in Fig 1) oscillation control port 340 In an exemplary embodiment of the invention, positive pressure 448 (ι e into port 340) in second oscillation control port 340 causes primary flow 332a to shift (270) to a second exit direction 360 (332b)

Now that primary flow 332b is in second exit direction 360, it creates (280) a negative pressure, optionally a stronger negative pressure, at the second oscillation control port and a direction of secondary flow (or pressure transmission) is reversed (290)

In an exemplary embodiment of the invention, this process is iteratively repeated and results in an oscillation of the primary flow between exit directions 350 and 360

Exemplary apparatus

Fig 1 is a schematic representation of an exemplary oscillating flow apparatus 300 according to one embodiment of the invention in lateral cross section The feedback tube is removed for clarity

In the depicted embodiment, an initial flow 312 enters an inner lumen of conduit 330 via a primary injection port 310 Optionally, port 310 includes a regulation mechanism adapted to vary a rate of initial flow 312 In an exemplary embodiment of the invention, varying initial flow

312 contributes to a change in an oscillation frequency In the depicted exemplary embodiment, area 432 of conduit 330 is adapted to mix flows 322 and 332

Optionally, initial flow 312 is supplemented by one or more suction flows 322 recruited via one or more suction ports 320 (two suction ports 320 are depicted, but any number is possible as long as symmetry is preserved) Optionally, ports 320 include a regulation mechanism adapted to vary a rate of suction flows 322 In an exemplary embodiment of the invention, varying suction flows 322 contributes to a change in an oscillation frequency Suction ports 320 can optionally be configured as slots

In an exemplary embodiment of the invention, initial flow 312 results from an energy input into the system Optionally, suction flows 322 are passively drawn into conduit 330 by flow 312 exiting injection port 310 according to Bernoulli's principle and entrainment process due to mixing between energetic flow 312 and entrained flows 322 Suction flows 322 mix with initial flow 312 to produce an amplified flow 332 (a or b) In an exemplary embodiment of the invention, amplified flow 332 is characterized by a flow rate representing a sum of flow rates of initial flow 312 and all suction flows 322 Optionally, suction flows 322 bring additional fluid into the system (e g from a boundary layer flowing past suction ports 320) In an exemplary embodiment of the invention, conduit 330 is sufficiently long to insure thorough mixing of initial flow 312 and suction flow(s) 322 before amplified flow 332 reaches oscillation control ports 340

In the depicted embodiment, amplified flow 332a proceeds through conduit 330 and is deflected by a splitter 370 towards exit direction 350 As amplified flow 332a passes oscillation control ports 340, flow 332a is closer to upper oscillation port 340 as in proceeds towards exit direction 350, as opposed to exit direction 360 In the depicted embodiment, exit directions 350 and 360 are separated by a splitter 370

Proximity of amplified flow 332a to upper oscillation port 340 causes a negative pressure 344 in upper oscillation control port 340, due to flow 332 turning around the corner The transmitted negative pressure 344 causes a series of cyclic oscillations of amplified flow 332 between exit directions 350 and 360 as described below with reference to Fig 1

As described above negative pressure 344 in upper control port 340 results from passage and turning of flow 332a across upper port 340 and into exit port 350

As depicted in Fig 1 , the negative pressure 344 creates a negative traveling pressure pulse (traveling opposite the arrows in oscillation feedback tube 440) which produces a positive flow 448 in lower oscillation control port 340

In an exemplary embodiment of the invention, the transmitted negative pressure pulse (or alternatively the positive flow) 448 deflects primary flow 332a from exit direction 350 to exit direction 360 where it is depicted as primary flow 332b

At this point, pressures in flow control ports 340 and the pressure sign in feedback control tube 440 are inverted

In an exemplary embodiment of the invention, primary flow 332 oscillates between an "a" state in which it flows towards exit direction 350 and a "b" state in which it flows towards exit direction 360 Optionally, an oscillation frequency (f) can vary with one or more of an offset distance 434 between a distal end of conduit 330 and a proximal end of an angled exit port (e g 350 or 360), a wall angle 436 of the angled exit port, a splitter distance 438 (between a distal end of conduit 330 and a proximal end of the splitter 370), a diameter 442 of oscillation port 340, a height 444 of oscillation port 340, a hydraulic diameter 446 of oscillation feedback tube 440 and a length 462 of an exit port (ι e linear distance from distal end of conduit 330 to egress from exit port (e g 360) without regard to angle) In an exemplary embodiment of the invention, as the wall angle 436 increases, a control pressure 344 and/or 448 at which oscillation begins also increases Exemplary oscillation rate control

Figs 4a and 4b are graphs illustrating exemplary characteristics of self-oscillating fluid flow apparatus according to some exemplary embodiments of the invention

In Fig 4a oscillation frequency (f in Hz) is plotted as a function of inlet flow rate (Q in Liters/sec) Data presented graphically in Fig 4a reflects experiments done with feedback tubes 440 having lengths of 80, 150, 250 and 450 cm and Q from 0 to 7L/s For a given tube length, f increases as Q increases For a given Q, a shorter feedback tube 440 produces a greater oscillation frequency (f) The effect of feedback tube 440 length on f becomes greater as Q increases

In Fig 4b oscillation frequency (f in Hz) is plotted as a function of feedback tube 440 length in meters Experiments were conducted with Q of 2 and 2 5 liters and oscillation tubes 440 with inner diameters 446 of 4 2 and 5 4 mm Results summarized graphically in Fig 4b indicate that for a given tube length and Q, f increases as tube diameter 446 increases Data presented in Fig 4b confirms again that f increases as Q increases and/or that f increases as tube length decreases In summary, Figs 4a and 4b indicate that as the dimensions of feedback tube 440 decrease, the oscillation frequency (f) increases In an exemplary embodiment of the invention, it is a volume of tube 440 which contributes to a change in f, although only diameter and length were experimentally tested Optionally, a volume of control ports 340 also influences f However, in practice, the combined volume of ports 340 is typically much smaller than a volume

of tube 440 Optionally, a control port 340 with a very small diameter 760 could negatively influence f even if tube 440 were characterized by a large volume In practice varying a dimension and/or volume of tube 440 and/or ports 340 is usually done in design or construction of the apparatus However, apparatus with mechanisms adapted for varying a dimension and/or volume of tube 440 and/or ports 340 after construction (e g during use) are within the scope of the invention

In an exemplary embodiment of the invention, a single apparatus is adapted to provide two or more different oscillation frequencies Optionally, all other geometry details and flow conditions remain fixed Optionally, two or more feedback tubes 440 of different lengths are connected in parallel between control ports 340 According to this exemplary embodiment of the invention, oscillation frequency (f) is determined by all unblocked feedback tubes 440 and their respective dimensions and/or volumes

In an exemplary embodiment of the invention, blocking one or more of the tubes changes f Optionally, this strategy is used to permit single apparatus to oscillate at a plurality of frequencies (f) In an exemplary embodiment of the invention, a desired range of frequencies is provided by constructing an apparatus with a suitable number of oscillation feedback tubes 440, each tube with characteristics to impart a desired frequency Alternatively, or additionally, variations in the frequency can be achieved by adjusting flow rate (Q) However, adjustment of Q also influences the magnitude flow 332 exiting exits 350 and/or 360 Fig 5 summarizes graphically results of an experiment performed using an apparatus with two feedback tubes 440 One feedback tube 440 had a length of 4 cm and the second oscillation tube 440 had a length of 9 5 cm Fig 5 is a plot of oscillation frequency (f in Hz) as a function of a rate of inlet flow 312 (Q in Liters/sec) using the 4cm tube (triangles), the 9 5 cm tube (circles) or the two tubes together in parallel (squares) Tubes not in use were only sealed at their center during the experiment but not removed Data presented in Fig 5 suggest that when two feedback tubes 440 are employed together, f is greater than when either tube is used individually, but that the f is less than a sum of the frequencies resulting from each tube individually

In an exemplary embodiment of the invention, one or more factors selected from inlet pressure, inlet flow-rate (Q) and output velocity contribute to oscillation frequency (f) during actuator operation

Since oscillation control pressures 344 and 448 are sensitive to back-pressure through exits 350 and/or 360, an area ratio between inlet nozzle 310 and conduit 330 at control port 340 (740) influences at which working conditions oscillation will occur as explained below Exemplary inlet port configuration

Fig 6 depicts an exemplary apparatus 700 in lateral cross section Apparatus 700 includes a converging-diverging inlet nozzle 710 with a cross sectional area at exit 712 of inlet nozzle 710 Optionally, nozzle 710 is adapted to produce an exit flow 732 as depicted in panel j of Fig 3

In an exemplary embodiment of the invention, a small cross-sectional exit area at exit 712 of inlet nozzle 710 can result in a high entrainment ratio of additional flow via suction ports 320 but also produces a high pressure-loss

Conversely a large cross sectional exit area 712 can result in a high output velocity (e g 332a or 332b) for a lower inlet pressure but can produce a low entrainment ratio In order to achieve a desired apparatus performance an area ratio should be carefully considered It has been experimentally determined that an area ratio defined as λ = A ₄ (at 740)/A-ι (at 712) in the range 3 to 15 encourage oscillation while λ outside this range can discourage oscillation In an exemplary embodiment of the invention, A ₄ (740) can be used as a primary engineering constraint in designing an apparatus of a desired size

Since parameters of the entire apparatus can be defined relative to a width 750 of the valve inlet duct 740, width 760 is typically held constant In order to change the switching valve inlet nozzle area without changing the nozzle width it is possible to change the inlet aspect ratio (depth ratio) σ = h/b (where h is the actuator depth (into the page in Fig 6) and b is width 750 of conduit 330 at a junction with oscillation control ports 340 In an exemplary embodiment of the invention, oscillation occurs when the aspect-ratio σ is between 1 5 and 10 A σ outside this range may serve to discourage oscillation

In the depicted exemplary lateral cross section of Fig 6, oscillation control ports 340 are not bilaterally symmetric with respect to their vertical axis Each of ports 340 terminates farther from a line extending from injection port nozzle 712 to an apex of splitter 370 on its right side than on its left side In the depicted embodiment, this termination is in a curve characterized by a corner radius 758 Termination in a sharp angle can cause undesired local flow separation In an exemplary embodiment of the invention, the corner radius r 758 is greater than 1 5 ^* b (750), and the splitter distance 438 is between 6 ^* b and 10 ^* b As splitter distance 438 decreases, oscillation tends to begin at higher control pressures 344 and 448 Exemplary multi-dimensional oscillation

Fig 6 depicts two oscillation control ports extending vertically in a Y direction as a means of causing flow 732 to oscillate between exits 350 and 360 which are vertically displaced from one another and angularly divergent In an exemplary embodiment of the invention, an additional pair of oscillation ports (not pictured), extend into and out of the page in a Z direction In an exemplary embodiment of the invention, these additional oscillation ports are connected by an additional oscillation control tube (not pictured for clarity) According to this additional exemplary embodiment, an additional pair of exits (not pictured for clarity), extend into and out of the page in a Z direction with a similar angular relationship as exits 350 and 360 have in the Y direction According to this exemplary multi-dimensional oscillation embodiment, flow 732 passing the four oscillation control ports will cause oscillation in the Y direction between exits 350 and 360 as described above, and also cause oscillation in the Z direction between the two additional exits In an exemplary embodiment of the invention, the Y and Z oscillations are out of phase so that flow

732 is cyclically directed to the 4 exits Optionally, the control ports and feedback tubes and exits in the Y and Z planes are characterized by similar dimensions

In an exemplary embodiment of the invention, 3 or 4 or more sets of oscillation control ports and feedback tubes are provided in different planes around an axis if conduit 330 Exemplary System

Fig 7 illustrates schematically an exemplary system 800 including an exemplary controller 810 and an exemplary apparatus 400 (depicted schematically as a dashed oval) of the general type described in detail above The depicted controller 810 includes one or more of a flow regulator 820, a switching mechanism 830 and a dimension changer 840 In an exemplary embodiment of the invention, flow regulator 820 regulates a flow rate through jet port 310 and/or suction ports 320 As described above in relation to Fig 4a, increased flow rate contributes to increased oscillation frequency

In an exemplary embodiment of the invention, switching mechanism 830 opens and/or closes two or more feedback tubes 440 As described above in relation to Fig 5, increased total volume of open feedback tubes 440 contributes to increased oscillation frequency

In an exemplary embodiment of the invention, dimension changer 840 alters one or more dimensions of feedback tube 440 and/or conduit 330 and/or control ports 340 Fig 5b illustrates, for example, that increasing length of feedback tube 440 contributes to decreased oscillation frequency Optionally, tube length is altered by slide valves (e g as employed in a trombone) or diversion valves (e g as in a trumpet)

In an exemplary embodiment of the invention, dimension changer 840 alters an effective diameter 446 of feedback tube 440

Fig 8 depicts one exemplary method of altering an effective diameter and/or length of feedback tube 440 In the depicted embodiment 900, control ports 340 are in fluid communication with two feedback tubes 440a (depicted as wide) and 440b (depicted as narrow) In an exemplary embodiment of the invention, valves (shown here as raise-able flaps

940a and 940b) are adapted to selectively close one or more of feedback tubes 440a and 440 b Optionally, valves 940a and 940b are controlled by switching mechanism 830 or are manually controlled In an exemplary embodiment of the invention, a control flow can be directed via ports 340 into one or both of feedback tubes 440a and 440b so that there are three possible effective diameters wide, narrow and wide + narrow Optionally, tubes 440a and 440b are a same or a different length

In an exemplary embodiment of the invention, concurrent closure of flaps 940a and 940b serves as an oscillation interruption mechanism for the apparatus Exemplary Use Scenarios

Various exemplary embodiments of the invention are expected to find utility in delay of boundary layer separation in aerodynamic and hydrodynamic applications Specific exemplary embodiments are lifting surfaces with high deflection angles (typically known as "high-lift

systems", aft bodies of helicopters and transport planes and aft regions in ground transportation systems (e g trucks, trailers, SUV's and/or trains)

In an exemplary embodiment of the invention, apparatus and methods described hereinabove are employed to reduce aerodynamic drag The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to necessarily limit the scope of the invention In particular, numerical values may be higher or lower than ranges of numbers set forth above and still be within the scope of the invention The described embodiments comprise different features, not all of which are required in all embodiments of the invention Some embodiments of the invention utilize only some of the features or possible combinations of the features Alternatively or additionally, portions of the invention described/depicted as a single unit may reside in two or more separate physical entities which act in concert to perform the described/depicted function Alternatively or additionally, portions of the invention described/depicted as two or more separate physical entities may be integrated into a single physical entity to perform the described/depicted function Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments can be combined in all possible combinations including, but not limited to use of features described in the context of one embodiment in the context of any other embodiment Specifically, features described in the context of a method can be used to characterize an apparatus and features described in the context of an apparatus can be used to characterize a method The scope of the invention is limited only by the following claims

In the description and claims of the present application, each of the verbs "comprise", "include" and "have" as well as any conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb

All publications and/or patents and/or product descriptions cited in this document are fully incorporated herein by reference to the same extent as if each had been individually incorporated herein by reference